gdb-gdb.py.in: Fix ordering of TypeFlags objects with Python 3

Python 3 doesn't use __cmp__ to order objects, it uses __lt__.  Because
of this, we get this exception when trying to pretty-print "type"
objects:

I tested it with Python 2.7 as well.

gdb/ChangeLog:

	* gdb-gdb.py.in (TypeFlag) <__cmp__>: Remove.
	<__lt__>: Add.